1. Field of the Invention
The present invention relates to PWM power regulators and/or DC-DC converters, and more particularly to a PWM switching DC-DC converter methodology for startup into a pre-charged load.
2. Description of the Related Art
The typical startup sequence of a pulse-width modulation (PWM) DC-DC converter is to ramp the non-inverting or positive “+” input of a feedback control error amplifier from zero (0) Volts (V) to a target VREF setpoint voltage level. The error amplifier is part of a feedback control loop which regulates an output voltage VOUT by comparing the reference voltage, VREF, with a fed back sensed portion of VOUT. In an exemplary buck-type converter, the PWM control circuitry drives the load through a switching device pair including an upper switching device and a lower switching device. The upper and lower switching devices are typically implemented as a MOSFET pair. If the converter is started into a precharged load, such as when VOUT is already charged, there can be large potentially damaging transient currents because VREF is less than the fed back sensed portion of VOUT. In particular, since the feedback loop attempts to regulate VOUT based on the voltage level at the input of the error amplifier, which is ramping up towards VREF, the “pull down” output switching device (e.g., the lower MOSFET of switching MOSFET pair) can be turned on long enough to exceed its thermal limit.